The present invention concerns the field of aeronautics. More specifically the invention relates to a temperature measurement method and temperature measurement system.
Some temperature probes of resistance type known as Resistance Temperature Detectors (RTD) have two electrically conductive sensing elements on one same substrate, each of the sensing elements being connected to a measuring channel to measure the resistance of said sensing element.
FIGS. 1 and 2 illustrate a common example of a said temperature probe 1. The temperature probe 1 has two similar sensing elements 3, 4 arranged on one same substrate 2. Each sensing element 3, 4 is powered by a channel 6, 7 of the Full-Authority Digital Engine Control (FADEC) via a current, so that the voltage at the terminals of said sensing element 3, 4 is able to be measured and the resistance of said sensing element 3, 4 computed. A first sensing element 3 is therefore connected to a first channel 6 of the FADEC 5, and a second sensing element 4 is connected to a second channel 7 of the FADEC 5.
FIG. 1 therefore illustrates dual sensing elements 3, 4 wound on a mandrel forming the substrate 2. The two sensing elements 3, 4 are wound in parallel around the mandrel. In general, the two sensing elements 3, 4 are platinum wires. FIG. 2 shows a probe of film type in which the sensing elements 3, 4 are bonded adjacent one another on the substrate 2.
The resistance of a sensing element 3, 4, in metal in particular and more particularly in platinum is dependent on the temperature of said sensing element 3, 4.
Therefore by determining the resistance of a sensing element 3, 4 through knowledge of the current circulating therein and of the voltage at its terminals, it is possible to deduce therefrom the temperature of said sensing element 3, 4.
However, the feed current creates power dissipation via Joule effect generating self-heating of the sensing element 3, 4 inside which the current circulates, leading to a self-heating error in temperature measurement. Since power dissipation is dependent on the current I and on the resistance R of the sensing element, corresponding to RI2, the feed current is chosen to be low, generally lower than 5 mA to limit these self-heating errors.
A self-heating error is also dependent on the mass flow rate of fluid e.g. air at the sensing element 3, 4, and hence at the temperature probe 1. FIG. 3 illustrates the influence of the mass flow rate of air (along the X-axis in Lbs/ft2·s) on self-heating error (along the Y-axis in ° C./mW) expressed as temperature error per milliwatt of power dissipated via Joule effect.
FIG. 3 gives a mean curve 20 surrounded by a high curve 21 and a low curve 22 respectively, reflecting the mean and the high and low dispersions respectively of self-heating error as a function of air mass flow rate.
For safety reasons, each sensing element 3, 4 in the event of failure must be able to withstand a current of 22 mA without destruction and with return to normal after correction of the failure. Said failure may originate from the FADEC computer power unit or from a resistance short-circuiting in the resistive path of the probe power supply.
Yet this type of common fault, having intensity much higher than the nominal current, causes strong self-heating via Joule effect of the sensing element 3, 4 in which said fault current circulates.
For example a fault current Ifault of 22 mA circulating in a sensing element 3, 4 having a resistance R of 200 ohms, induces dissipated power Pdissipated of:Pdissipated=R×Ifault2 Pdissipated=200×0.0222 Pdissipated=0.0968 W−100 mW
By applying the mean curve 20 illustrated in FIG. 3, for an air mass flow rate of 6 lbs/ft2·s, a dissipated power Pdissipated of 100 mW gives a mean error of 0.03×100=3° C. An error of 3° C. lies above an acceptable global error threshold, typically of 1.1° C.
Since the two sensing elements 3, 4 are positioned on one same substrate, the self-heating of one of the sensing elements 3, 4 is likely to propagate via thermal conduction to the other sensing element 3, 4. For example for a wound probe such as illustrated in FIG. 1, the parallel windings of the sensing elements 3, 4 are very close, in the order of about 60 μm. The temperature of the other sensing element 3, 4, in which there circulates a normal feed current, is therefore increased leading to an error in the temperature measurement of the fluid. As a result the measurement of one sensing element 3, 4 may be affected by a fault affecting the other sensing element 3, 4.
Therefore, in the event of an ordinary fault on the computer side which only affects one channel, the fault will be propagated to the entire probe 1 whose two channels 6, 7 will have the defect of an unacceptable error. Yet the duality of the sensing elements 3, 4 precisely has the objective of guaranteeing the availability of reliable temperature measurement even in the event of ill-functioning on one channel 6, 7 or one sensing element 3, 4.
There is therefore a risk of losing the two temperature measurement channels subsequent to an ordinary fault which should only affect one channel. Loss of air temperature measurement may lead to degraded engine performance even to stalling of the engine.
The measurement by said temperature probe is used in numerous engine control laws such as the position of the variable geometries of the high pressure compressor driven at low speed as per:
      Xn    ⁢                  ⁢    25    ⁢                  ⁢    R    =            Xn      ⁢                          ⁢      25                                temperature          ⁢                                          ⁢          measurement                288.15            Xn25 being the speed of the high pressure rotor of said compressor in revolutions per minute. Since control over transitory engine speeds has recourse to measurement of air temperature obtained with the temperature probe, the accuracy of such measurement is most important.
Various solutions have been proposed to overcome these disadvantages. FIG. 4 for example shows a probe 30 comprising two mandrels 31, 32, a sensing element 33, 34 being wound around each of the mandrels 31, 32. The sensing elements 33, 34 are therefore separate, the distance between them being sufficient so that a self-heating error at one sensing element 33, 34 does not cause a recovery error at the other sensing element 33, 34 via thermal conduction.
Such configuration has several drawbacks however. There is a distance between the sensing elements 33, 34, which may lead to differences in temperature measurement possibly preventing combined use of the two temperature measurements in some cases. In addition, a said probe 30 is more voluminous, heavier, has a higher drag coefficient (Cx) and hence a wider wake.